The amulet blasts were only so massive and powerful in DLOTS because Kun couldn't control them
Really? Before Kun even mentions anything about "not being able to control it", he says "Yes, Nadd...I...I believe I begin to understand", and the omniscient narrator comes in to say "What Exar Kun understands is that the dark rage that fills his own heart can be focused in this amulet, unleashing tremendous energies!" - and on the same page we see him use the blasts which seem to have the same magnitude as the others. Then, on the next page, he says he can't control it as the energy increases - though there is no mention of this on the previous page of Kun not controlling it when we see the blasts first disintegrate temple walls.
and the amulet was feeding directly of Kun's power.
Kun can use "the dark rage that fills his own heart" - his anger - and focus it in the amulet, different from the amulet "feeding off Kun's power".
When he learns how to control them (and it is clear that he does indeed learn how to control them when he says to Aleema that she is a sith pretender and that he has leant everything from the ancient sith
(whether he did or didn't, it is clear that he believed that he had) so it is presumable that he learnt how to control the amulet blasts),
It's presumable to believe he actually knew how to control the blasts, when it isn't clear [to you] that he has learned everything of "Sith powers"?
the blasts are nothing that special as is shown when he uses them against Aleema. It is a single beam, hardly more powerful then a simple force power and Aleema is shortly able to get up unhurt.
The blast had enough power to throw her across the room, and knock her unconcious - but not enough to even leave a burn? I highly doubt that's the same attack. Not ruling out it isn't, but I wouldn't count it as the same either. Even though they had two different artists, the attacks look different anyways - red squiggly lines and pink DBZ blasts.
As well, have you taken into account that he wasn't even attuned to the Darkside during that point? Hadn't learned jack of the Darkside, and was almost clueless?
The fact he could use blasts to that magnitude, direct them, and not be consumed by them when he didn't know jack about the Darkside is impressive. Since that time he's gained immense knowledge - "more knowledge and wealth than he can ever hope to use", vast insights into the Darkside, and obviously tremendous power - to think he can't produce the same effect and control it would call for an actual explanation rather than "omg Aleema blastz = fin4l product".
I suppose though that it's the same attack (not really) - I mean Ulic, who knew nothing of Nadd's direct teaching and barely anything of the amulet he possessed, knew how to or did create blasts like Kun did on Yavin, but instead "learned everything", and used a blast which had similiar effect to Kun's blast on Aleema, on Cay and Nomi, right?
Oh, and Akbar - your sig is awesome.